1. Field of the Invention
This invention relates to a magnetic thrust bearing and more particularly a magnetic thrust bearing that uses permanent bias flux with a simplified construction to allow for highly efficient force generation, high speed rotation capability and low cost construction.
2. Description of Related Art
Magnetic thrust bearings were originally constructed by using a single ferromagnetic disk attached to a rotating shaft. The thrust disk is then acted upon by electromagnets with a C-shaped cross-section located above and below the disk. This offers a very simple and low cost
construction but has a very low efficiency along with requiring complex nonlinear control.
The next advancement uses the same mechanical construction, however the electronics employs a large constant current to each coil to generate a bias flux. A small control current is added on top of the bias currents to control the bearing. The result of using a bias flux is simplified control because the relationship of force to control current becomes linearized. Linearization is provided because the force is proportional to the square of the flux density. This functions by adding the control flux to one coil""s bias flux and at the same time subtracting the same control flux from the other. The force generated is then directly related to the difference of the squares of the net top and net bottom fluxes, and this varies linearly with control current. The drawback of this bearing configuration is the steady-state electrical inefficiency from having to electrically maintain the bias currents. FIG. 1 shows the configuration 30. The thrust disk 32 is attached to the shaft 31 and acted upon by an upper C-core ring 34 and a lower C-core ring 33. An upper coil 36 and lower coil 35 are used to generate magnetic flux. A bias current is applied to each coil 35, 36 to generate bias fluxes 37 and 38. A control current is then applied in superposition to the bias currents in each coil 35, 36 which generates control fluxes 39 and 40. In FIG. 1, the upper control and bias fluxes 40, 38 add and the lower control and bias fluxes 39, 37 subtract. The net result is force exerted upward on the disk 32 that varies linearly with the control current. A non-dimensionalized example on the linearization is as follows. If the bias fluxes have an arbitrary value of 5 and a control flux is superposed with a value of 1, the flux on the top side of the disk becomes 6 and on the bottom side becomes 4. The net force is then (6{circumflex over ( )}2xe2x88x924{circumflex over ( )}2) or 20. Because of the bias flux, the relation of force to control current becomes both linearized and amplified. With a control flux of 2, the resulting force would then be double, 40. Without the bias flux, control flux would only be applied to one core at a time to generate force and a control flux of 1 and 2 would result in forces of only 2 and 4. Two amplifiers would also be required for operation.
An improved design of magnetic thrust bearings places permanent magnets in series with the electromagnets so that the bias flux is generated without use of electric power. U.S. Pat. Nos. 3,937,148 and 5,003,211 show variations using this concept. This design increases the steady-state electrical efficiency, however the permeability of high energy permanent magnets is very low. Therefore, the electromagnets require much more control current to generate the same control flux because of the higher reluctance of the magnetic circuits. FIG. 2 shows the configuration 50. The thrust disk 52 is attached to the shaft 51 and is acted upon by an upper C-core ring 54 and a lower C-core ring 53. Permanent magnets 57 and 58 generate the bias fluxes 59 and 60. Opposed control currents in coils 55 and 56 generate the control fluxes 61 and 62. As before, the control and bias fluxes are additive in one core 54 and subtractive in the other 53, resulting in an upward force on the disk 32. Unfortunately, magnets 57 and 58 have permeability comparable to an airgap. Therefore, the required control current to generate the equivalent control fluxes 61 and 62 as in the configuration 30 control fluxes 39 and 40 is much higher.
A further improvement is to use permanent magnets for generating bias flux but the permanent magnet flux paths are made non-coincident with the path of the electromagnet flux. The permanent magnets are not in series with the electromagnets but instead share only a portion of the same paths that include the airgaps. The result is a greatly improved design that allows for both linear and highly efficient control. U.S. Pat. No. 3,890,019 is one configuration and this is shown in FIG. 3. The thrust disk 72 is attached to the shaft 71 and is acted upon by a single external C-core yoke ring 73. A single coil 74 is used to generate the control flux 79. Permanent magnets 75 and 76 generate the bias fluxes 77 and 78. Superposition of the control and bias fluxes 79,77,78 cause an upward force on the disk 72. The only drawback with this configuration is that it does not achieve the highest possible force capability or efficiency because of ill-defined large airgaps in the permanent magnetic flux paths 77 and 78.
U.S. Pat. No. 3,865,442 is a more efficient design using the same concept of non-coincident control and bias flux paths. FIG. 4 shows the configuration 130. Three thrust disks 132, 133, 134 are attached to the shaft 131. The thrust disks 132, 133, 134 are acted upon by a single external C-core ring 135 with a control coil 136 for producing control flux 141. Permanent magnets 137 and 138 attached to the shaft 131 generate the bias fluxes 139 and 140. The drawbacks of this design are the use of rotating permanent magnets, which limit the high speed rotation capability due to their low strength, and the complexity. The use of three thrust disks is also undesirable.
U.S. Pat. No. 3,955,858 discloses an improved thrust bearing design in which the permanent magnet is stationery. The configuration 90 is shown in FIG. 5. Two thrust disks 92 and 93 are attached to the shaft 91 and are acted upon by stator rings 94 and 95. A radially magnetized permanent magnet 96 generates the bias flux 99. The control flux 100 is generated by the control coils 97 and 98. As shown, superposition of the fluxes results in an upward force on the disks 92 and 93. The design unfortunately has a more complicated than desired construction, including a radially magnetized permanent magnet and two thrust disks. U.S. Pat. No. 5,315,197 describes the same configuration but also discloses a modified version, allowing for use of only one thrust disk. The drawback to this design is the inclusion of a radial airgap in the magnetic circuit. The radial airgap causes generation of radially destabilizing forces. A similar configuration, U.S. Pat. No. 5,514,924, adds multiple radial control coils to the same design.
U.S. Pat. No. 5,250,865 shows further improved thrust bearing configuration by only requiring one thrust disk and all permanent magnets are stationery. Unfortunately, the invention is complicated and requires use of four permanent magnets with eight airgaps. The bearing also requires assembly of multiple precision pieces for generation of the five flux paths.
More recently, U.S. Pat. No. 5,804,899 discloses a magnetic bearing with a biased thrust actuator. This invention is same thrust bearing as disclosed in U.S. Pat. No. 5,317,197 but only with a large structure added and some separate permanent providing some radial centering force. A radially magnetized permanent magnet and two thrust disk portions are again required.
There still exists a need for a high force, high efficiency magnetic thrust bearing that can allow for high speed rotation and also has a simple, low cost construction
The invention is an improved magnetic thrust bearing that uses permanent magnets to provide bias flux. The magnetic circuits of the control flux and bias fluxes are substantially non-coincident but they do share the same path over some portions which include axial airgaps. This allows for a low reluctance and efficient path for the electromagnets flux. The flux paths of the permanent magnets are completely defined with minimized airgaps for achieving higher forces and efficiency and very low control currents produce extremely large forces. The design uses a single coil and amplifier for simplicity and only a single thrust disk is required. Likewise, no radially magnetized permanent magnets are required and no permanent magnets are attached to the rotor that would require reinforcement.
Specifically the present invention is an electromagnetic bearing for a thrust member having a distal region extending outwardly from a support comprising: at least one ferrous member, such as an upper and lower yoke having a coil, the ferrous member straddles the distal region of the thrust member, confronting surfaces 123, 124, 125, 126 of at least two extrusions of the ferrous member and thrust member defining control flux air gaps on opposite sides of the thrust member, and generating an electromagnetic control flux path through the air gaps whereby to axially position the ferrous member relative to the thrust member; confronting surfaces 127, 128, 129, 130 of at least one permanent magnet and either the thrust member or the ferrous member, defining at least one magnetic air gap spaced from at least one of the control flux air gaps, and generating a bias flux path parallel and non-coincident with the control flux path for a substantial portion of its length, wherein the permanent magnet is outside the control flux path and the length of each air gap in said bearing is limited to the physical separation of the confronting surfaces.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following detailed description, appended claims, and accompanying drawings